Update `servcheck validate`

[chbrandt]

ServCheck validate option uses Astropy VO validator engine. The engine being used by EADA has been deprecated as pointed by:

WARNING: AstropyDeprecationWarning: The check_conesearch_sites function is deprecated and may be removed in a future version.
        Use astroquery.vo_conesearch.validator.validate.check_conesearch_sites instead. [__main__]
WARNING: AstropyDeprecationWarning: The VOSDatabase class is deprecated and may be removed in a future version.
        Use astroquery.vo_conesearch.vos_catalog.VOSDatabase instead. [astropy.vo.client.vos_catalog]
WARNING: AstropyDeprecationWarning: The VOSBase class is deprecated and may be removed in a future version.
        Use astroquery.vo_conesearch.vos_catalog.VOSBase instead. [astropy.vo.client.vos_catalog]